A person recognizes the direction of and distance to a sound source from the difference between the sound heard by the left and right ears. The difference between the sound heard by the left and right ears arises from the different distances from the sound source to the left and right ears, that is, the different characteristics (frequency characteristics, phase characteristics, loudness, etc.) imprinted on the sound as it propagates through space. By intentionally imparting a difference in these characteristics to a sound-source signal, it is possible to have the signal recognized as coming from an arbitrary direction and distance. A head related transfer function (HRTF) is a well-known way to represent the characteristics acquired by a sound source during propagation to the ears. By measuring the HRTFs froth a virtual sound source to the ears and then imparting these characteristics to a signal, it can be made to seem that a sound is being heard from the virtual sound source. In principle the virtual sound source may be disposed at any location, provided HRTFs can be obtained for all points in space, but this is impractical because of restrictions on structural size, such as the amount of hardware. To deal with this problem, in the ‘virtual sound source control server’ described in Non-Patent Document 1, many HRTFs are obtained from few HRTFs by interpolation.
Non-Patent Document 1: Yasuyo YASUDA and Tomoyuki OYA, ‘Reality Voice and Sound Communication Technology’, NTT Technical Journal (NTT Gijutsu Janaru), Vol. 15, No. 9, Telecommunications Association, September 2003.